Small tube amp conversion help 6x4, 6AU6, 6AQ5

[getonemaker]

Hi Everyone,

I have bought a small tube amp from Ebay. It is converted to a guitar amp from some other application. With a DOD 250 pedal and an 8 Ohm speaker it is nice, but without the pedal it is not loud for my guitar. Please look at the schematics and advise mods that would make it a bit louder.

The Electrolytic caps are 350V 30 uF and 20 uF, but don't know which one is which. I hope there is an easy fix.

Also, could I put a "tone block" between the tubes, like before the coupling capacitor (such as at in http://www.netads.com/~meo/Guitar/Amps/PA2Guitar/)?

Thanks!

[Jazz P Bass]

Measure the Vac at the speaker output while you are playing the amp.
Square that measurement & divide it by the speaker resistance.
That is the wattage of the amp.
A 6AQ5 is good for 5-6 watts at 350 volts.
That is all & that is it.
http://www.mif.pg.gda.pl/homepages/f...093/6/6AQ5.pdf

[RWood]

Two things I would do to that circuit to make it a little louder are 1) remove the negative feedback circuit (or at least try it without it). Thats the line that comes off the OT secondary (yellow wire) back to the cathode of the input tube.

And 2) get rid of everything on the input /grid circuit except a 22K ~ 68K grid blocker resistor and a 1M resistor to ground. Then move the volume control so that it's between the input tube and the output tube. Look at a 5F2-A volume / tone for an example.

Okay, maybe 3) things. The screen wiring on the input tube looks strange (pin 6) It needs a higher value screen resistor and then a capactor to ground. I don't have a circuit in mind to copy there but if you really want to get the most out of a pentode input, read the chapter in Merlin's book and tweak those values. Simpler yet, ditch the pentode and go with a 12AX7. The benefit will be that you then have a million circuits from which to reference.

In my opinion, what you need here is to copy a known circuit while you gradually figure out what each thing does.

RWood

ps The 30uF cap would be the one closest to the rectifier, then the 20. It won't have an effect on your volume though.
Your link has an extra )? at the end of it

[RWood]

I didn't mean to discourage you from keeping with the tubes you've got. Here's the Merlin stuff, which he has generously placed online:
The Valve Wizard -Small Signal Pentode
In the example he uses an EF86, which is a 9 pin pentode more frequently found in guitar amps than the 6AU6 which is more of a phono / radio tube. But they both have the same basic elements and there is a lot of good info there for you.

Here is an example in action, with Mark Lavelle's Decimator. Check out how he sets up the 6AU6,
which shows what I was describing in my first response. Here is the rest of his site:
Harmonic Appliances (Mark Lavelle)
Where his 6AU6 goes to a volume control then to the input grid of the next gain stage, yours would go to the grid of your power tube.

RWood

[Enzo]

Right away I find it very unlikely that the output tube has a 330 ohm resistor to ground from its grid, and a 270k cathode resistor. I might believe if those two were swapped.

Also, I doubt that 10Meg resistor at the input goes to the tip lead of the jack. Much more likely it goes to ground.

[Gtr_tech]

Noticed that right off. 330 on the cathode and 270k for grid gnd ref....

[getonemaker]

Correction and some mods

Thank you all for your replies! I apologize for some incorrect readings. The 10M resistor WAS soldered to the Vol pot...
I took some advise and here is the current status of the amp. By removing the feedback and changing the 33 nF to a 20 mF the amp got somewhat louder, but still not 3-4W, I think. Also, now has a much nicer tone.

Can you suggest some additional mods? What would happen if I change the coupling capacitor to 0.1 mF? Someone did that in a similar circuit to get more gain.

Thanks again!

[getonemaker]

We're in this together and ever

[Gtr_tech]

Coupling caps don't affect overall gain per se, ( gain IS reduced at lower freq's) but freq response. Larger values let more low end thru, and thats not usually a good thing. The tone can get really muddy and flabby in a hurry. If yer goin' for a good overdrive, then you really need to keep the low end in check. That was the whole idea behind the Dallas Rangemaster. It was essentially a boost with a lot of low end rolloff. So then when you dimed an amp with this in front of it, you got a smoother distortion without the flabby low end.